Various dispensing systems have been used in the past for applying patterns of viscous liquid material, such as hot melt adhesives, onto a moving substrate. In the production of disposable diapers, incontinence pads and similar articles, for example, hot melt adhesive dispensing systems have been developed for applying a laminating or bonding layer of hot melt thermoplastic adhesive between a nonwoven fibrous layer and a thin polyethylene backsheet. Typically, the hot melt adhesive dispensing system is mounted above a moving polyethylene backsheet layer and applies a uniform pattern of hot melt adhesive material across the upper surface width of the backsheet substrate. Downstream of the dispensing system, a nonwoven layer is laminated to the polyethylene layer through a pressure nip and then further processed into a final usable product.
In various known hot melt adhesive dispensing systems, continuous filaments of adhesive are emitted from a multiple adhesive outlet die with multiple process air jets oriented in various configurations adjacent the circumference of each adhesive outlet. The multiple air jets discharge air generally tangentially relative to the orientation of the discharged adhesive filament or fiber as the filament emerges from the die orifice. This process air can generally attenuate each adhesive filament and cause the filaments to move back and forth in overlapping or non-overlapping patterns before being deposited on the upper surface of the moving substrate.
Manufacturers of diaper products and others remain interested in small fiber technology for the bonding layer of hot melt adhesive in nonwoven and polyethylene sheet laminates. To this end, hot melt adhesive dispensing systems have incorporated slot nozzle dies with a pair of angled air channels formed on either side of the elongated extrusion slot of the die. As the hot melt adhesive emits from the extrusion slot as a continuous sheet or curtain, pressurized process air is emitted as a pair of curtains from the air channels to impinge upon, attenuate and fiberize the adhesive curtain to form a uniform fibrous web of adhesive on the substrate. Fibrous web adhesive dispensers have incorporated intermittent control of adhesive and air flows to form discrete patterns of fibrous adhesive layers with well defined cut-on and cut-off edges and well defined side edges.
Meltblown technology has also been adapted for use in this area to produce a hot melt adhesive bonding layer having fibers of relatively small diameter. Meltblown dies typically include a series of closely spaced adhesive nozzles or orifices that are aligned on a common axis across the die head. A pair of angled air channels or individual air passages and orifices are positioned on both sides of the adhesive nozzles or orifices and align parallel to the common nozzle axis. As hot melt adhesive discharges from the series of aligned nozzles or orifices, pressurized process air is discharged from the air channels or orifices and attenuates the adhesive fibers or filaments before they are applied to the moving substrate.
While meltblown technology has been used to produce fibrous adhesive layers on moving substrates, it has various areas in need of improvement. As those skilled in the art will appreciate, meltblown technology typically uses a high volume of high velocity air to draw down and attenuate the emitted adhesive filaments. The high velocity air causes the fibers to oscillate in a plane that is generally aligned with the movement of the substrate, i.e., in the machine direction. To adequately blend adjacent patterns of adhesive to form a uniform layer on the substrate, meltblown dispensers require the nozzles to be closely spaced. Moreover, the volume and velocity of the air must be high enough to sufficiently agitate and blend adjacent fibers.
However, the high volume of air used in conventional meltblown dispensers adds to the overall operational cost as well as reduces the ability to control the pattern of emitted fibers. One byproduct of the high velocity air is “fly” in which the fibers get blown away from the desired deposition pattern. The “fly” can be deposited either outside the desired edges of the pattern, or even build up on the dispensing equipment which can cause operational problems that require significant maintenance. Another byproduct of the high velocity air and closely spaced nozzles is “shot” in which adjacent adhesive fibers become entangled and form globules of adhesive on the backsheet substrate. “Shot” is undesirable as it can cause heat distortion of the delicate polyethylene backsheet.
It will be further appreciated by those skilled in the art that when typical meltblown dies are placed in side-by-side fashion across the width of a moving substrate a less consistent fiber pattern on the substrate results. This occurs since each meltblown die has continuous sheets of air formed on either side and these sheets of air are interrupted between adjacent meltblown dies.
Other air-assisted nozzles or dies use capillary style tubes mounted in a nozzle or die body for extruding filaments of thermoplastic material. Air passages are provided adjacent to the tubes, and the ends of the tubes project outwardly relative to the outlets of the air passages.
Various forms of laminated plate technology are known for extruding rows of adhesive filaments in an air assisted manner. These include dispensing nozzles or dies constructed with slotted plates for discharging filaments of liquid and process or pattern air for attenuating and moving the discharged filaments in a desired pattern. These nozzles or dies present various issues relating to their performance, design complexity and large numbers of plates needed to complete the assembly. Therefore, improvements remain needed in this area of technology.